1. Field of Invention
This invention relates to methods for producing hydrofluorocarbon compounds. More specifically, this invention relates to isomerization of hydrofluoroalkenes.
2. Description of Prior Art
Currently, there is a worldwide effort to develop hydrofluorocarbons (HFCs), or compounds containing only carbon, hydrogen and fluorine, for applications as refrigerants, blowing agents, solvents and diluents for gaseous sterilization. Unlike chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), both of which potentially damage the Earth's ozone layer, HFCs contain no chlorines and thus pose no threat to the ozone layer. In this regard, trans-1,3,3,3-tetrafluoropropene (trans-1234ze) is a compound that has the potential to be used as a zero Ozone Depletion Potential (ODP) and a low Global Warming Potential (GWP) refrigerant, blowing agent, aerosol propellant, solvent, etc., and also as a fluorinated monomer.
Methods for producing HFO-1234ze are known. For example, U.S. Pat. No. 5,710,352 teaches the fluorination of 1,1,1,3,3-pentachloropropane (HCC-240fa) to form HCFO-1233zd and a small amount of HFO-1234ze. U.S. Pat. No. 5,895,825 teaches the fluorination of HCFO-1233zd to form HFO-1234ze. U.S. Pat. No. 6,472,573 also teaches the fluorination of HCFO-1233zd to form HFO-1234ze. U.S. Pat. No. 6,124,510 teaches the formation of cis and trans isomers of HFO-1234ze by the dehydrofluorination of HFC-245fa using either a strong base or a chromium-based catalyst in the presence of an oxygen containing gas. U.S. Pat. No. 7,563,936, which is incorporated herein as a reference, teaches the isomerization of cis-1234ze into trans-1234ze over judiciously selected isomerization catalysts. U.S. Pat. No. 7,485,760, incorporated herein as a reference, further discloses an integrated process to produce trans-1234ze from 245fa, which comprises: (a) dehydrofluorinating 1,1,1,3,3-pentafluoropropane to thereby produce a result comprising cis-1,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene and hydrogen fluoride; (b) optionally recovering hydrogen fluoride from the result of step (a); (c) isomerizing at least a portion of the cis-1,3,3,3-tetrafluoropropene into trans-1,3,3,3-tetrafluoropropene; and (d) recovering trans-1,3,3,3-tetrafluoropropene. In one preferred embodiment of that disclosure, 245fa dehydrofluorination and cis-1234ze isomerization are combined in one reactor vessel and after HF is recovered trans-1234ze is isolated as purified product from the overhead of a distillation column and the mixture of unconverted 245fa and cis-1234ze collected as heavies in the reboiler is recycled back to the reactor.